1. Technical Field
The present invention relates to making differential impedance measurements of a resistive device using a reflection bridge. More particularly, the present invention relates to a reflection bridge providing for differential measurements of both 50 Ohm and 100 Ohm complex impedances.
2. Related Art
Conventional differential measurement systems for measuring 100 Ohm components included balanced 100 Ohm transmission lines. Characterization of components was provided using multiple single port measurements. The balanced parameters were extracted from at least two single ended measurements.
FIG. 1 shows a conventional differential bridge used to make differential measurements by obtaining two single ended measurements. The test system includes a signal source 2 connected through a 50 Ohm line 4 to provide an input. A resistive load 8, labeled R1, and resistive load 9, labeled R2, are each connected to one of two test ports TEST PORT 1 and TEST PORT 2 to be measured. A measurement output OUT is provided from one terminal of a resistor 6, with a second end of resistor 6 connected to ground. Two couplers 10 and 12 individually connect the test ports with the signal source 2 to one of the test ports, and return a reflected signal to the output OUT. The couplers 10 and 12 are individually connected with switches, the first coupler 10 being connected with the switches in the “1” position, and the second coupler 12 being connected with the switches in the “2” position.
The two separate measurements at the output OUT determine θ1 and θ2. The values for θ1 and θ2 are in turn used to provide a combined differential measurement θDIFF. The measurements of θ1 and θ2 are made using two single ended 50 Ohm S11 measurements between two test ports TEST PORT 1 and TEST PORT 2. With RL=R1+R2, and RL=100 Ohms, a 100 Ohm differential S11 measurement is effectively obtained. Complex math is needed to determine the differential measurement value θDIFF using the equations shown to follow.
          ⁢                                        ⁢                                    θ              =                                                R                  -                  50                                                  R                  +                  50                                                      ,                          R              =                              50                ⁢                                  (                                                            1                      +                      θ                                                              1                      -                      θ                                                        )                                                                                                            ⁢                                    R              DIFF                        =                          R1              +              R2                                                                              ⁢                                    R              DIFF                        =                                          50                ⁢                                  (                                                            1                      +                      θ1                                                              1                      -                      θ1                                                        )                                            +                              50                ⁢                                  (                                                            1                      +                      θ2                                                              1                      -                      θ2                                                        )                                                                                                            ⁢                                    θ              DIFF                        =                                                            R                  DIFF                                -                100                                                              R                  DIFF                                +                100                                                                                            ⁢                                    θ              DIFF                        =                                                            50                  ⁢                                      (                                                                  1                        +                        θ1                                                                    1                        -                        θ1                                                              )                                                  +                                  50                  ⁢                                      (                                                                  1                        +                        θ2                                                                    1                        -                        θ2                                                              )                                                  -                100                                                              50                  ⁢                                      (                                                                  1                        +                        θ1                                                                    1                        -                        θ1                                                              )                                                  +                                  50                  ⁢                                      (                                                                  1                        +                        θ2                                                                    1                        -                        θ2                                                              )                                                  +                100                                                        
More recently a 50 Ohm to 100 Ohm balanced transformer was introduced to alleviate some of the problems encountered using the two measurement extraction approach. The balanced transformer approach, however, has limitations due to the need of a broad band 1:√2 turns ratio. The turns ration is provided by a balanced transformer, which is the heart of the converter.
FIG. 2 shows components of a differential bridge used such a balanced transformer 20. The differential bridge uses the signal source 2 and resistor 4 as provided through a coupler 10, as in FIG. 1. Unlike in FIG. 1, the balanced transformer 20 is used to connect the coupler 10 output to both test ports TEST PORT 1 and TEST PORT 2 in FIG. 2 so that an additional coupler and switches as in FIG. 1 are not needed. The output θ is provided from the coupler 10 through a resistor 6 connected to ground. The common mode current does not appear due to the “floating” secondary TEST PORT 2 replacing ground connections. Components carried over from FIG. 1 to FIG. 2 are similarly labeled, as will be components carried over in subsequent drawings.
The transformer 20 transforms from 50 Ohms to 100 Ohms with a turns ratio of 1:√2 turns ratio, or √(100/50)=1.4142. The transformer 20 is difficult to construct because of the non-integer or half-integer value of the turns ratio. A practical transformer typically requires a turns ratio ranging from 1 to 1.5. The RL to RP value would be 1.52 or 2.25. Using the specially constructed transformer to provide the turn ratio shown, a single test measurement of θ can be used to determine the resistance RL using the following formulas:
                            ⁢                  RP          =                                    RL                              N                2                                      =                                          RL                                                      2                                    2                                            =                              RL                2                                                                                    ⁢                  θ          =                                                    RL                2                            -              50                                                      RL                2                            +              50                                                                      ⁢                  θ          =                                    RL              -              100                                      RL              +              100                                          
The 50 Ohm bridge would be balanced with 50 Ohms at its input and 50*2.25=112.5 Ohms between the differential test port outputs TEST PORT 1 and TEST PORT 2. Because the RL typically cannot be 112.5, post processing can correct for this error after an open-short-load (OSL) calibration.
FIG. 3 shows components of a conventional basic measurement bridge. The bridge is balanced when the load resistor 8, labeled RL, is 50 Ohms and the output OUT is at 0 Volts. The bridge of FIG. 3 uses resistors 31–33 as opposed to one of couplers 10 or 12 of FIG. 1 to provide reflected test signals from the test port to the output OUT for measurement. A transformer 30 (labeled T1) connects the signal source 2 and resistor 4 to the bridge formed by resistors 31–33. The T1 transformer 30 has a first winding connecting the resistor 4 to ground. A second winding of he T1 transformer 30 connects the TEST PORT and one end of resistor 31 to a common node 25. The common node 25 is provided between resistors 32 and 33. The common connection of resistors 31 and 32 form the output port OUT. The output port OUT is where test measurements are obtained, and is connected from the resistor 51 through a resistor 6 to ground.
Only a single ended measurement of the load resistor RL is available from TEST PORT 1, so multiple measurements must be made to obtain a differential measurement, similar to that described with respect to FIG. 1. Further, with a ground path through resistor 6, the bridge is subject to common mode impedance disturbances.
FIG. 4 shows modification of FIG. 3 to provide a transmission line balun 30A. A first winding of T1 balun 30A connects from the input IN to the TEST PORT. A second winding of the T1 balun 30A connects from the common node 25 to ground. The transmission line balun 30A can be provided using a standard 50 Ohm transmission line with ferrite beads placed over it. The ferrite beads extend of the balun characteristics to low frequencies. This allows operation of the transformer T1 from the low megahertz to the tens of gigahertz.
It would be desirable to provide a device to measure 100 Ohm components using a standard 50 Ohm system without requiring a two measurement process, and without requiring use of a balanced transformer.